Controlled Wind Speed to Wind Load as per ASCE 7-16

[Cfx0522]

Hi everyone, I am currently working on a project related to canopy structure. Due to the strength and flexibility of the structural members, the canopy will not be able to resist the wind load calculated as per basic wind speed as per ASCE 7-16, and thus a wind sensor will be installed to monitor and make sure the screens are retracted at a certain wind speed. When calculating the wind load, there are many K factors involved in the equation based on surface roughness, elevation, etc... and the basic wind speed is nominal design 3-s gust wind speed at 33 ft above ground for Exposure Category C.

My question is: If a wind sensor is installed to retract screen or shade at a wind speed lower than the basic wind speed as per ASCE 7, what value should I take for all the K factors if the exposure category and elevation is different? Should I take 1.0 for all of them (might be too conservative)? Or should I still take the value as per the clauses and tables to calculate wind load?

Thanks for the help.

 

[canwesteng]

If the sensor is detecting wind speed at the same elevation as the screen, probably all you need to adjust for is elevation above sea level for air density. It would be unconservative to use the 3s gust speed as the speed to pull down the shade though, you should try and relate it to some nominal "sustained" wind speed.

 

[driftLimiter]

Why dont you measure the wind pressure instead of the wind speed and take out all of the other variables.

 

[EZBuilding]

Chapter 31 of the FBC may be of some auxiliary use to you.

 

[Cfx0522]

driftLimiter. I think the motor that control the roof screen is based on the wind speed determined by the wind sensor.

 

[driftLimiter]

Wind design pressure vs velocity is far from exact. Look at the range of the values of the K factors and make a judgement. Selecting 1.0 for everything is inherently conservative.

 

[MotorCity]

You will have to design for the worst case.....maybe it acts like an open structure with the wind shade up? maybe it acts like an enclosed structure with the wind shade down?......or maybe somewhere in between (a partially open or partially enclosed structure). Unfortunately, the code is not going to cut you a break on the wind load because your structure is equipped with a wind sensor (which could fail during a power outage). This is similar to checking a building with a larger overhead doors. Depending on if the doors are open or closed during a wind event could significantly affect the wind load on the structure.

 

[phamENG]

Makes sense that they'd base it on wind speed - anemometers are cheap.

My concern is in the reliability of this control circuit. What fail safe does it have? Does it do any self checking? Who's job is it to maintain it and test it? How frequently will that happen? A design wind event may occur the day after it's installed, or it may occur 30 years after it's installed - will this control circuit still be functioning properly? In making that circuit part of the way you're satisfying the code requirements, you're making it part of your structure and it needs to have comparable reliability to what the structure is required to have.

You also need to know how the anemometer is reporting data and how the control circuit is processing and using that data. The anemometer itself is likely going to feed real time, instantaneous wind speed data. That's essentially the 3-s gust speed (last I checked, nobody was confident in saying their sensors were reporting data faster than that with sufficient reliability/accuracy). But what is the control circuit doing? Is it smoothing it out to a 1 or 2 minute average wind speed? I hope so, because that's what you'll want. You want your system to trigger when there is a high probability of a damaging gust occurring. So you need to figure out what your canopy can handle, and then back calculate the approximate average wind speed that would be happening to see that 3-s gust. (Instructions for that are in ASCE 7 commentary.)

I'd say it would be best to design it for at least the 10 year MRI wind gust speed, and then have a 'standing order' to retract the canopy if the forecast calls for winds above whatever speed (again, keep in mind that wind forecasts are usually 1 or 2 minute averages, not gusts). Hopefully the mechanism will retract it, but if not a person knows to do it. If they fail to do it...well hopefully the wreckage doesn't hurt anyone.

Make sure the AHJ understands what's being done and that they agree in writing. Otherwise, you'll still be the one who failed to design the structure to code - wind speed based retraction mechanism or not.